1. Field of the Invention
This invention relates to a shutter device for a camera, and particularly to a shutter device for a still camera.
2. Related Background Art
A stop and shutter (hereinafter referred to as a lens shutter) carried in a conventional compact camera, particularly a lens shutter carried in a high-class compact camera, is of such a construction as shown in FIG. 8A of the accompanying drawings.
In FIG. 8A, two shutter blades 31a and 31b are supported for rotation about shafts 32a and 32b, respectively, and a protruding shaft 34 provided on a lever 33 is fitted in slots 33a and 33b (the slot 33b being hidden and unseen behind the shutter blade 31a) formed in the respective shutter blades 31a and 31b. The lever 33 is mounted on an actuator 35, and the actuator 35, the lever 33 and the protruding shaft 34 together constitute driving means.
The actuator 35 is of the moving coil type and is rotatable in the direction of arrow 36 and therefore, the protruding shaft 34 is pivotally moved in the direction of arrow 37 through the intermediary of the lever 33, and the shutter blades 31a and 31b are driven in the directions of arrows 38a and 38b, respectively, by the slots 33a and 33b in which the protruding shaft 34 is fitted, thereby limit a light beam in the direction of the optical axis 39.
The shutter blade 31b is provided with a position indicating plate 310 which, in turn, is formed with a plurality of slits 311. The reference numeral 312 designates a photointerrupter which counts the number of the slits 311 passing through a space 313, thereby detecting the position of the shutter blade 31b, and thus detecting the positions of the two shutter blades 31a and 31b, i.e., the limited amount of the light beam in the direction of the optical axis 39, because the shutter blades 31a and 31b are driven substantially equal amounts by the protruding shaft 34 fitted in the slots 33a and 33b. The reference numeral 314 denotes the ground plate of the shutter, and the reference numeral 315 designates an opening (aperture) formed in the shutter ground plate.
In the above-described construction, the output of the photointerrupter 312 is input to the microcomputer 316 of a camera, a driving circuit 317 is controlled by the output of the microcomputer 316, and the actuator 35 is driven as controlled. The photometric value of an object from photometering means 318 is also input to the microcomputer 316, and the driving circuit is controlled by the photometric value and the output of the photointerrupter 312.
The method of control is such that for example, when the object is very light, the microcomputer 316 drives the shutter blades 31a, 31b in an opening direction by the pulse number of the pulse output of the photointerrupter 312 corresponding to the photometric value of the photometering means 318, and thereafter drive the shutter blades 31a, 31b in a closing direction, and when the object is dark, the microcomputer 316 drives the shutter blades 31a, 31b in the opening direction until the pulses of the photointerrupter 312 are all output (the shutter blades 31a, 31b become fully open), and continues to drive the shutter blades 31a, 31b in the opening direction still after that. In this case, the shutter blades 31a, 31b are fully open and are not driven as controlled any more in the opening direction by a stopper or the like, not shown, and remain fully open. After the lapse of a predetermined time, i.e., a sufficient time to expose the object to film, the shutter is driven in the closing direction to thereby fully close the shutter blades 31a, 31b.
Supplementing the above description of the operation, the shutter blades 31a, 31b are initially in their fully closed state, and by a shutter button being released, the shutter blades 31a, 31b are driven in the opening direction. Then, for example, when the object is light, the shutter is driven in the closing direction by a small pulse number, e.g. the two-pulse output of the photointerrupter 312 accompanying it, and when the object is dark, the shutter is driven in the closing direction after the lapse of a predetermined time after all the pulses of the photointerrupter 312 are output.
FIG. 8B of the accompanying drawings shows the flow of the driving operation of the shutter device of FIG. 8A. At a step S801, the photometric value of the object is input and on the basis thereof, at a step S802, the shutter opening diameter (aperture value) is found, and a corresponding count pulse n is obtained (the shutter is fully closed when n=0, and the shutter is fully open when n=n.sub.0). Subsequently, on the basis of the input photometric value, at a step S803, a waiting time t is found. At a step S801, the shutter is electrically energized in the opening direction, whereby the shutter begins to be opened and a pulse is generated from the photointerrupter. At a step S805, when n pulses are generated, shift is made to the next step S806, and waiting is done for t seconds, whereafter at a step S807, the shutter is electrically energized in the closing direction, whereby the shutter is closed, thus terminating the operation. When the object is light, n is a small value and t is 0 or a very short time, and when the object is dark, n=n.sub.0 and t is a long time. Therefore, when the object is light, the optical path is limited by the shutter blades 31a, 31b (the aperture area is small) and the opening time 41a is short, as shown in FIG. 9A of the accompanying drawings, and when the object is dark, the shutter is fully open and the opening time 41b becomes long.
In FIGS. 9A to 9I, the axis of abscissa represents time and the axis of ordinate represents the aperture area created by the shutter blades 31a, 31b being driven in the opening direction.
Here, consider a case where the photographer actually effects photographing. For example, in the case of a situation such as when a person who is the main object is to be photographed with a mountain as the background, the photographer often wants to take a photograph in which both the person and the mountain are in focus. When such a photograph is to be taken, it is necessary to stop down the aperture (reduce the shutter aperture area) as much as possible as shown in FIG. 9C and keep the shutter open until a sufficient time to expose the film. However, such a shutter blade driving method cannot be realized by the example of the prior art shown in FIGS. 8A and 8B, because in the example of the prior art shown in FIGS. 8A and 8B, the shutter can be driven only in the opening or closing direction and cannot be kept in a stopped-down state (a state of small aperture area).
In FIG. 8A, it seems that the shutter can be kept in the stopped-down state if in order to keep the shutter in the stopped-down state, for example, the shutter blades 31a, 31b begin to be opened and the shutter is operated in the closing direction after the photointerrupter 312 outputs two pulses, and the shutter blades continue to be driven in the closing and opening directions in conformity with an increase or a decrease in the pulses thereafter, but actually, the photointerrupter 312 can detect the number (pulses) of the slits 311 which have passed therethrough, but cannot know the direction in which they pass (an increase or a decrease in the pulses). For example, the photointerrupter cannot discriminate between a case where even if the shutter is electrically energized in the opening direction and after two pulses are output, the shutter begins to be electrically energized in the opposite direction, the shutter still moves in the opening direction due to its inertia and one pulse is output and then the shutter begins to be driven in the closing direction and -1 pulse is output and a predetermined desired aperture diameter is attained, and a case where the shutter is electrically energized in the opening direction and two pulses are output, whereafter the shutter begins to be electrically energized in the opposite direction and the shutter still moves in the opening direction due to its inertia and two pulses are output. That is, the photointerrupter cannot know the direction in which the shutter is being driven and therefore, the shutter cannot be kept in its stopped-down state. To keep the aperture in its stopped-down state, there is available a system as used in a video camera wherein the difference between the quantity of light incident on an image pickup element 51 shown in FIG. 10 of the accompanying drawings and a command value 52 is amplified by an amplifying circuit 53 and the shutter blades 31a, 31b are driven by the output of the amplifying circuit 53. In this case, the image pickup element 51 can detect the quantity of light incident thereon and the direction of increase or decrease in the quantity of light and therefore, can keep the aperture in a state in which the quantity of light incident on the image pickup element 51 is optimum.
Of course, it is impossible in terms of space and cost to prepare an image pickup element 51 in a still camera, but if from a similar point of view, as shown in FIG. 11 of the accompanying drawings, a hole 61 gradually widening along the opening direction of the shutter blade 31b is formed through a position indicating portion 310, the output of the photointerrupter will be varied by the position of the shutter blade 31b, and the position and direction of driving of the shutter blade 31b can be known from that output and the shutter blade 31b can be held by that output. That is, when in FIG. 11, the shutter blades 31a, 31b are driven in the opening direction, the output of the photointerrupter 312 becomes greater, but when the output of the photointerrupter 312 is smaller than the output of the microcomputer 316 conforming to the aperture value (aperture area) determined by the photometric value of the photometering circuit 315, the output difference is amplified by the amplifying circuit 53 and the driving circuit drives the driving means to thereby open the shutter blades 31a, 31b.
As the output of the photointerrupter 312 becomes approximate to the output of the microcomputer 316, the driving force of the driving means driving the shutter blades 31a, 31b in the opening direction becomes weak, and when the output of the photointerrupter 312 becomes equal to the output of the microcomputer 316, the driving force in the opening direction becomes null. However, the shutter blades 31a, 31b move further in the opening direction due to their inertia forces and therefore, the output of the photointerrupter 312 becomes greater than the output of the microcomputer 316. Thereupon, the polarity of the output difference is now reversed and therefore, the driving circuit 317 drives the driving means in the opposite direction via the amplifying circuit 53 and drives the shutter blades in the closing direction. When the output of the photointerrupter 312 again becomes smaller than the output of the microcomputer 316, the shutter blades 31a, 31b are again driven in the opening direction.
By such repetition, the shutter blades 31a, 31b become gradually stabilized into a predetermined aperture value determined by the output of the microcomputer 316. When the output of the microcomputer 316 becomes small after the lapse of a predetermined exposure time, the shutter blades are driven in the closing direction so that the output of the photointerrupter 312 may become small (that is, the shutter blades may be closed), whereby the shutter is closed. If an attempt is made to drive the shutter blades 31a, 31b as shown, for example, in FIG. 9C, the output of the microcomputer 316 can be put out at a low level for a long time as shown in FIG. 9F, and if the shutter blades are driven as shown in FIG. 9B, the output of the microcomputer 316 can be put out at a high level as shown in FIG. 9E, and if the shutter blades are driven as shown in FIG. 9A, the output of the microcomputer 316 can be put out at a low level for a short time as shown in FIG. 9D, and the method of driving the shutter blades can be freely controlled by the magnitude and output time of the output of the microcomputer.
The output of the microcomputer itself (the output in the microcomputer) can be output only as a binarized value and therefore, even if the output interval .delta.t is varied as shown in FIGS. 9D, 9E, 9F, 9G, 9H and 9I and is passed through a conventional smoothing circuit to thereby create the outputs of FIGS. 9D, 9E and 9F, the numerical data in the microcomputer can be converted into an analog output by a D/A converter to thereby create the outputs of FIGS. 9D, 9E and 9F.
A technique of detecting the amount of driving as shown in FIGS. 10 and 11 and automatically driving the shutter blades so as to decrease the difference between the value thereof and a target value is called an automatic control technique, and such a technique is used in a variety of fields.
However, an attempt to control the shutter aperture area (aperture) precisely in the example of the prior art shown in FIG. 11 has led to the following disadvantages.
There is an individual difference in the sensitivity output of the photointerrupter 312, and when for example, in FIG. 11, the output of the photointerrupter when the shutter is fully closed is V.sub.1 and the output when the shutter is fully open is V.sub.2 and the difference therebetween is V.sub.3, assuming that the photointerrupter has been replaced by a new one and the output thereof when the shutter is fully closed is V.sub.11 and the output thereof when the shutter is fully open is V.sub.22 and the difference therebetween has become V.sub.33, V.sub.1 and V.sub.11, V.sub.2 and V.sub.22, and V.sub.3 and V.sub.33 do not coincide very much with each other. Also, V.sub.1, V.sub.2 and V vary considerably with temperature and time. This has led to the disadvantage that even if the output of the photointerrupter is controlled so as to assume a target value, the actual aperture area varies with the individual difference, temperature and time and a correct aperture diameter is not obtained.
Also, in the shutter operation as in the prior art (the shutter operation described above in connection with FIGS. 8A and 8B) as well as during the stop-down slow shutter mode (i.e., when the aperture is stopped down so that both of the object and the background may be in focus, and long shutter time exposure is effected), the amount of opening is not controlled accurately and the exposure onto the film becomes "over" or "under".